1. Field of the Invention
The present invention relates to, for example, radio communication. In particular, this invention relates to detection of a beginning timing of a received signal in a receiving section of a radio communication apparatus.
2. Description of the Related Art
In the field of radio communications, there is known IEEE802.11 standards defined by the Institute of Electrical and Electronics Engineers (IEEE). A receiving section of a system based on the standards will now be described in brief.
FIG. 9 is a functional block diagram schematically showing a general structure of a receiving section of a radio communication apparatus based on IEEE802.11 standards. Signals received by antennas 102a or 102b are frequency-converted and amplified by a radio section 103. An output signal from the radio section 103 is amplified with a predetermined gain by an automatic gain control (AGC) section 105. An output signal from the AGC section 105 is converted to a digital signal by an analog-to-digital (A/D) converter section 106. An output signal from the A/D converter section 106 passes through a band filter section 107 and an automatic frequency control section 108 and is then demodulated into a data bit string by a demodulation process section 109.
A signal head detection section 110 detects a beginning timing of the received signal. A control section 112 controls antenna diversity in the radio section 103. The control section 112 also adjusts the gain in the AGC section 105 so that the amplitude of the input signal to the A/D converter section 106 may fall within a suitable conversion range of the A/D converter section 106.
In radio communications based on IEEE802.11 standards, etc., a preamble, which precedes a data signal, is added to the head of a radio signal. The preamble has a form in which a predetermined number of known patterns, each having a certain cycle, are combined. The preamble is used for signal head detection, automatic gain control, antenna diversity control, timing synchronization, automatic frequency control, etc.
In order to effect antenna diversity, the control section 112 finds mean power values of preambles of reception signals received by the antennas 102a and 102b. In consideration of characteristics of the preambles, a certain time period is needed to find the mean values of the received power.
In addition, timing synchronization and automatic frequency control are performed using preambles, it is necessary that the antenna diversity control be finished prior to these operations. In short, the antenna diversity control has to be completed at early stages of preambles.
Since there are many processes associated with preambles, it is necessary to detect as early as possible the beginning timing of preamble signals, i.e. the heads of the received signals.
In a conventional signal head detection method, for example, power mean values of signals are obtained for two different periods and the beginnings of the signals are detected using a difference between the mean values as well as a reference value. In addition, it is generally known that noise other than desired signal(s) mixed in received signals. In order to eliminate the effect of noise, an average time for finding power mean values is increased. In particular, when a signal-to-noise ratio (SNR) is low, it is necessary to obtain average values over a long time. Hence, it is difficult to detect heads of received signals at an early stage.
Furthermore, a certain time period is needed until the gain control of the AGC section 105 is stabilized after the control section 112 delivered a control signal to the AGC section 105. Consequently, the heads of received signals cannot be detected until this control is completed.
The gain of the AGC section 105 is fixed while reception signals are being awaited, that is, until the heads of received signals are detected. Besides, the conversion range of the A/D converter section 106 is limited. Because of these factors, signals out of the conversion range may be supplied to the A/D converter section 106, depending on the levels of received signals. As a result, an output signal of the A/D converter 106 may be saturated. In particular, when the noise levels of the received signals are high and the noise level of the output signal of the A/D converter section 106 is nearly saturated, power mean values of desired signal(s) would hit the ceiling. Then, the difference between power mean values decreases below the reference value and the heads of the received signals cannot be detected.